Lithium-thionyl chloride cells are particularly advantageous sources of electrochemical power. Such electrochemical cells provide reasonably flat discharge curves at temperatures down to about -30.degree. C., with only very slight voltage delay at low current densities. Moreover, it is well known that because of the unique properties of lithium--it has the highest electrode potential of any element, while at the same time, it is the lightest metallic element--lithium battery systems afford exceptional capacity in proportion to the weight of the anode.
In lithium-thionyl chloride cells of the prior art, a porous carbaceous material, usually a carbon black and Teflon mixture is used as the cathodic current collector. The electrolyte, usually a combination of a lithium halide with a Lewis acid, is dissolved in the cathodic liquid depolarizer, i.e., the thionyl chloride. The cathodic side of the cell is protected from the lithium anode by a porous separator stable in thionyl chloride, such as non-woven fibrous glass, fibrous aluminum oxide or other non-conductive, porous materials which are chemically stable in lithium-thionyl chloride cells.
Such prior art lithium-thionyl chloride cells have found many practical utilities, including the power source for cardiac pacemakers. Other practical applications, which have taken advantage of the high energy density and the favorable low temperature characteristics of lithium-thionyl chloride cells include balloon and rocket borne meteorological radiosondes, emergency locating transmitters, underwater instrumentation and powering cryogenic experiment. However, such prior art lithium-thionyl chloride cells often present safety problems, especially when cells are used serially in batteries. It is well known and documented that under conditions of voltage reversal, lithium-thionyl chloride cell can undergo electrochemical and chemical reaction which cause cell bulging, excess heating, or even explosions. In U.S. Pat. No. 4,307,160, the explosive force which can occur in lithium-thionyl chloride cells during voltage reversal is likened to an "incendiary bomb."
This problem encountered during cell reversal is believed to be caused by the formation of lithium dendrites which plate across the separator to the surface, or into the mass, of the porous carbon current collector thereby creating a electrical pathway between the lithium and the carbon. The intermixing of the lithium and the carbon, upon the application of current during cell reversal, generates excess heat which can cause a precipitous rise in internal cell pressure. Additionally, the presence of elemental sulfur and sulfur dioxide, the by-products of chemical and electrochemical reactions in the cell, also contribute to the generation of heat and the increase in internal cell pressure. Importantly, as pointed out in U.S. Pat. No. 4,307,160, voltage reversal will take place when the batteries are connected in series and may occur even though reasonable precautions seemingly have been used.
A well known solution to the problem of explosions in lithium--thionyl chloride electrochemical cells is to provide for a safety vent in the cell housing. While such vents may prevent the cell from exploding under voltage reversal conditions, in many situations venting is unacceptable since the environment around the cell will become contaminated by the highly toxic fumes.
Accordingly, it is an object of the present invention to provide an improved lithium-thionyl chloride cell which will not undergo electrochemical and chemical reactions under voltage reversal conditions. It is a further object of the present invention to provide for a mechanism within a lithium-thionyl chloride cell which, during voltage reversal will prevent the explosion of the cell while preventing the contamination of the surrounding atmosphere. Yet another object of the present invention is to provide for a lithium-thionyl chloride cell which is safe under cell reversal conditions, yet easy to manufacture. The foregoing and additional objects will become more fully apparent with the following description.